Threaded connections for pipes are well known in the gas and oil industry. The pipes to which the connections are applicable include those which are used in both the exploration for and the transport of fluid materials. Because of the nature of the materials and the pressures at which the pipes operate it is important that the seal in the joints between adjacent pipe sections remains effective. Such joints can be difficult to maintain due to the forces to which the length of the pipe is normally subjected. The forces can, on a particular joint, be compressive or tensive along the axis of the pipe or both tensive and compressive for example, when the pipe is in a curved configuration. The forces, together with pressure differentials across the width of the pipe, can weaken a seal and create a pathway for the fluid within the pipe to leak out.
In the oil and gas industries, it is obviously desirable to extract from all parts of the reserve being exploited. In recent years with the advent of improved joints there has been a move away from the previous practice of introducing new substantially vertically orientated bore holes. It is now well known to run a pipeline horizontally through the reserve. In doing this, a portion of a previously drilled bore hole can be used. Such a method however requires the introduction of substantial curvature into the pipeline in order to go from being vertically directed to horizontally directed. The individual joints between pipes must therefore be able to withstand the resultant compressive force.
In order to improve metal-to-metal seals in the connections, a number of common features are typically included as part of the joint. Firstly, the threaded portion of the joint is angled with respect to the main axis of the pipe sections. Secondly, a frusto-conical, non-threaded shoulder is normally included at the end of the male section of pipe (the male section often being referred to as the pin) with a corresponding recess in the female section (often referred to as the box). The shoulder can also include curved portions, chamfers and other features to improve the strength of the resulting seal.
Often in order to further strengthen the joint a corresponding seal is provided on the outer surface of the pipe joint between the box section and the pin section. Such outer joints usually suffer the disadvantage that the crest height of the screw-thread diminishes towards the outer edges of the pipe to accommodate the shoulder. This lesser crest height reduces the effectiveness of the outer seal.
It is therefore an object of the present invention to provide a seal which seeks to address the above problems and provide a pipe joint having an improved inner and outer seal.